1. Field of the Invention
This invention relates to a question answering system, a data search method, and a computer program, and more particularly to a question answering system, a data search method, and a computer program, which can provide a more precise answer to a question in a system wherein the user enters a question sentence and an answer to the question is provided.
2. Description of the Related Art
Recently, network communications through the Internet, etc., have grown in use and various services have been conducted through the network. One of the services through the network is search service. In the search service, for example, a search server receives a search request from a user terminal such as a personal computer or a mobile terminal connected to the network and executes a process responsive to the search request and transmits the processing result to the user terminal.
For example, to execute search process through the Internet, the user accesses a Web site providing search service and enters search conditions of a keyword, category, etc., in accordance with a menu presented by the Web site and transmits the search conditions to a server. The server executes a process in accordance with the search conditions and displays the processing result on the user terminal.
Data search process involves various modes. For example, a keyword-based search system wherein the user enters a keyword and list information of the documents containing the entered keyword is presented to the user, a question answering system wherein the user enters a question sentence and an answer to the question is provided, and the like are available. The question answering system is a system wherein the user need not select a keyword and can receive only the answer to the question; it is widely used.
For example, JP 2002-132811 A discloses a typical question answering system. JP 2002-132811 A discloses a question answering system configuration including a question analysis section, an information inspection section, an answer extraction section and a ground presentation section. The question analysis section determines a search word (search keyword) set and the question type from a question sentence presented by the user. The information inspection section makes a search based on the search word (search keyword) set to extract a passage as a sentence including the search word. The answer extraction section extracts several answer candidates from the passage. The ground presentation section presents the ground of the answer candidates.
In such a question answering system, the answer extraction section performs a processing of extracting an answer corresponding to the user question from among the search results; it is not easy to precisely extract only the answer corresponding to the user question from among a large number of search results obtained from the information inspection section.
Then, a technique of inspecting the extracted answer candidate based on hypothesis that a positional relationship between the search word and the answer candidate appearing in a sentence in a passage has a feature is developed. For example, “Is Question Answering an Acquired Skill?” (Ganesh Ramarkrishhnan, Soumen Chakrabarti, Deepa Paranjpe, Pushpak Bhattacharya, in proceedings of the 13th international conference on World Wide Web, pp. 111-120 (2004)) discloses the processing configuration wherein the nearness of a distance between the clauses of the search word and the answer candidate contained in the passage is learned with a decision tree and an answer candidate is selected.
“Question Answering System with Graph Structure from Dependency Analysis” (KURATA Gakuto, OKASAKI Naomi, ISHIZUKA Mitsuru, Information Processing Society of Japan Report NL-158, pp/69-74 (2003)) discloses the processing configuration wherein the results of conducting syntactic analysis on sentences in a passage are graphed and the calculation result of the distance between the search word and the answer candidate contained in the graph is used for ranking the answer candidates.
As described above, if the knowledge sources to be searched, such as a database and a Web page, are searched based on a user question and the extracted passage (sentence group containing search keyword) contains an answer fitted to the question, it is difficult for the question answering system of the related art to efficiently extract an appropriate answer required by the user.
As described above, each of “Is Question Answering an Acquired Skill?” and “Question Answering System Using Graph Structure Based On Modification Relationship” discloses the processing configuration for selecting the answer candidates. Of the related art, in “Is Question Answering an Acquired Skill?”, if a sentence has a complicated structure made up of compound sentences, complex sentences, or a large number of modifiers, the clauses containing the search word and the answer candidate tend to be at a distance from each other; this is a problem. In this case, if measurement of the distance between clauses is learned, the appropriate relationship between the search word and the answer candidate is not extracted, and thus an erroneous learning result is output; this is a problem.
In “Question Answering System Using Graph Structure Based On Modification Relationship”, the sentence set in the corresponding passage is graphed collectively and the distance is calculated and thus it is impossible to take out the precise relationship between the search word and the answer candidate in comparison with distance calculation in sentence units and it is difficult to appropriately rank the answer candidates; this is a problem.